1. Field of the Invention
The present invention generally relates to a method and apparatus for shaping a workpiece with a shaping electrode by intermittently impressing a pulse to generate a spark across a working gap formed between the workpiece and the electrode, and more particularly to a method and apparatus for shaping a workpiece wherein the width of the voltage pulse is controlled according to the state of the working gap.
2. Description of the Prior Art
In the past, in processes for shaping a workpiece by electrical discharge, the state of the working gap was naturally variable, and often resulted in an unusual spark across the gap and thereby damaged the workpiece and electrode if the electrical condition, such as the mean value of the discharge current, was left uncontrolled. The operator therefore had to adjust the electrical condition as the state of the working gap changed. To this end, the operator had to have a considerable amount of experience and skill in finding an optimum electrical condition. FIGS. 1 to 4 describe an electrical discharge apparatus of the prior art.
FIG. 1 schematically illustrates an electrical discharge shaping electrode 1 positioned adjacent a workpiece 2 through a working gap. The electrode 1 and the workpiece 2 could be immersed in an insulating fluid such as kerosene. The references 3A, 3B, . . . ., 3N denote a plurality of transistors, connected in parallel to each other, which generate square-wave voltage pulses to spark an intermittent current across the working gap. The number of transistors used depends upon the amount of discharge current required. Hence, for example, one transistor will suffice when the discharge current required is low. The references 4A, 4B, . . . ., 4N indicate resistors for controlling and balancing the currents flowing through the collectors of the transistors. Resistors 5A to 5N are base resistors for controlling the currents flowing through the bases of the transistors, and a timer 6 comprises a pulse generating circuit including such elements as an astable multivibrator, a monostable multivibrator and a flip-flop circuit. An amplifier 7 amplifies pulses generated in the timer 6 and supplies an output to the transistors 3A, 3B, . . ., 3N. The numeral 8 designates a DC power source.
FIGS. 2 and 3 show voltage and current waveforms appearing across the working gap of the apparatus in FIG. 1. The waveforms in FIG. 2 appear when the pulse width and a quiescent period are controlled, and those in FIG. 3 appear when the duration of discharge is controlled by increasing the pulse width as a function of the period during which the no-load voltage is present across the working gap. In FIGS. 2 and 3, the numeral 9 denotes a pulse width, 10 a quiescent period, 11 a period during which the no-load voltage is impressed, 12 a duration of discharge, 13 a no-load voltage, 14 a discharge voltage, 15 a discharge current, 16 a peak discharge current, and 17 a mean processing current. Under stable processing conditions, the no-load voltage 13 appears with a high probability, and its mean period 11 is controlled to be constant by a servo mechanism capable of maintaining constant the means processing voltage across the working gap. This control, however, is stable only when the state of the working gap is good. In other words, if the state of the working gap has deteriorated due to, for example, a deposit of chips between the working gap, the no-load voltage impressing period diminishes or even vanishes as shown in FIG. 4(a) relative to FIGS. 2 and 3. As a result, it is very likely that the discharge will be concentrated at a specific point, and thus will produce a hollow portion on the workpiece.
In such an event a current waveform as shown in FIG. 4(b) appears, the duration of a discharge becomes larger than those shown in FIGS. 2(b) and 3(b), and the mean current increases accordingly. If this condition continues for a certain period of time, an extinction of ions across the working gap does not properly occur, the discharge is concentrated at one point on the workpiece and the state of the working gap is further impaired. To solve this problem, the state of the working gap must be restored by decreasing the mean current.
One prior art method of decreasing the mean current was to detect the state of the working gap in terms of discharge current mean value, and to change the oscillation frequency according to the detected value. While somewhat satisfactory, this method, however, was not practical because the oscillation frequency could not be changed fast enough in response to the quickly varying state of the working gap. Accordingly, this method was not a very effective method for decreasing the mean current.